Technical Field
Embodiments of the invention relate generally to power generation and, more particularly, to an apparatus for mitigating heat loss through the stack of a heat recovery steam generator during outage periods.
Discussion of Art
Gas turbines have been widely used to provide electric power, usually as a standby for both peak power and reserve power requirements in the utility industry. Gas turbines are preferred because of their rapid starting capability and low capital cost. Conventional gas turbines, however, operate with reduced thermal efficiency due to the high exit temperatures of the exhaust gas stream and the resulting thermal loss. Therefore, a gas turbine is often combined with a heat recovery steam generator to improve overall system efficiency.
As is known in the art, heat recovery steam generators can be employed to drive a steam turbine for power output, such as in a combined cycle power plant, or to provide process steam in cogeneration cycles. For example, flue gas discharged by the gas turbine may be conducted through a heat recovery steam generator for the generation of steam and then discharged to atmosphere through a stack.
During outage periods of a power plant, such as during night or weekend shutdowns, heat may be lost from the heat recovery steam generator through the stack due to natural draft. This heat loss through the stack also results in a decrease in pressure within the boiler drums of the heat recovery steam generator that must be recovered during the following start up, which increases the fuel cost and the time required to bring the plant back online and operating at full capacity.
In addition to the increase in fuel cost and time required to transition the plant back to full capacity after a shut down, frequent shut down and start up can contribute to cyclical stresses in the components of the heat recovery steam generator. In particular, cyclical stresses may result from changes in the temperature of components as the plant is shut-down and re-started. Because of the high stream pressures involved, many of the components are thick-walled so the temperature changes do not occur evenly across the components. This results in differing rates of thermal expansion and contraction across the component and high material stresses.
Existing strategies to reduce thermal stresses during shut down and subsequent re-start include the use of metallic dampers within the stack that can be selectively closed during shutdown. Such dampers help to prevent heat loss from the heat recovery steam generator through the stack during shut down periods, thereby reducing the magnitude of temperature changes and thus thermal stresses within the components of the heat recovery steam generator, as well as decreasing the time required and amount of fuel needed to bring the plant back up to full capacity. Many building and safety codes, however, prevent such metallic stack dampers from being retrofit into existing heat recovery steam generator stacks due to the immense weight of such dampers. Indeed, retrofitting of such dampers may often require a complete redesign of the stack and/or foundations to accommodate the stack damper weight.
In view of the above, there is a need for system, method and apparatus for quickly and easily sealing off the stack of a heat recovery steam generator to avoid natural draft during shut down periods of a power plant, and which system and apparatus may be retrofit into existing heat recovery steam generator stacks without requiring significant modifications to the stack or foundation.